Light emitting diodes (LEDs) are semiconductor-based light sources often employed in low-power instrumentation and appliance applications for indication purposes. LEDs conventionally are available in a variety of colors (e.g., red, green, yellow, blue, white), based on the types of materials used in their fabrication. This color variety of LEDs recently has been exploited to create novel LED-based light sources having sufficient light output for new space-illumination applications. For example, as discussed in U.S. Pat. No. 6,016,038, multiple differently colored LEDs may be combined in a lighting fixture, wherein the intensity of the LEDs of each different color is independently varied to produce a number of different hues. In one example of such an apparatus, red, green, and blue LEDs are used in combination to produce literally hundreds of different hues from a single lighting fixture. Additionally, the relative intensities of the red, green, and blue LEDs may be computer controlled, thereby providing a programmable multi-color light source. Such LED-based light sources have been employed in a variety of lighting applications in which variable color lighting effects are desired.
For example, U.S. Pat. No. 6,777,891 (the “'891 patent”), incorporated herein by reference, contemplates arranging a plurality of LED-based lighting units as a computer-controllable “light string,” wherein each lighting unit constitutes an individually-controllable “node” of the light string. Applications suitable for such light strings include decorative and entertainment-oriented lighting applications (e.g., Christmas tree lights, display lights, theme park lighting, video and other game arcade lighting, etc.). Via computer control, one or more such light strings provide a variety of complex temporal and color-changing lighting effects. In many implementations, lighting data is communicated to one or more nodes of a given light string in a serial manner, according to a variety of different data transmission and processing schemes, while power is provided in parallel to respective lighting units of the string (e.g., from a rectified high voltage source, in some instances with a substantial ripple voltage).
The operating voltage required by each lighting unit (as well as the string, due to the parallel power interconnection of lighting units) typically is related to the forward voltage of the LEDs in each lighting unit (e.g., from approximately 2 to 3.5 Volts depending on the type/color of LED), how many LEDs are employed for each “color channel” of the lighting unit and how they are interconnected, and how respective color channels are organized to receive power from a power source. For example, the operating voltage for a lighting unit having a parallel arrangement of respective color channels to receive power, each channel including one LED having a forward voltage on the order of 3 Volts and corresponding circuitry to provide current to the channel, may be on the order of 4 to 5 Volts, which is applied in parallel to all channels to accommodate the one LED and current circuitry in each channel. Accordingly, in many applications, some type of voltage conversion device is desirable in order to provide a generally lower operating voltage to one or more LED-based lighting units from more commonly available higher power supply voltages (e.g., 12 VDC, 15 VDC, 24 VDC, a rectified line voltage, etc.).
One impediment to widespread adoption of low-voltage LEDs and low-voltage LED-based lighting units as light sources in applications in which generally higher power supply voltages are readily available is the need to convert energy from one voltage to another, which, in many instances, results in conversion inefficiency and wasted energy. Furthermore, energy conversion typically involves power management components of a type and size that generally impede integration. Conventionally, LEDs are provided as single LED packages, or multiple LEDs connected in series or parallel in one package. Presently, LED packages including one or more LEDs integrated together with some type of power conversion circuitry are not available. One significant barrier to the integration of LEDs and power conversion circuitry relates to the type and size of power management components needed to convert energy to the relatively lower voltage levels typically required to drive LEDs.
For example, voltage conversion apparatus (e.g., DC-to-DC converters) typically utilize inductors as energy storage elements, which cannot be effectively integrated in silicon chips to form integrated circuits. Inductor size is also a serious barrier to integrated circuit implementations, both in terms of an individual inductor component as part of any integrated circuit, as well as more specifically in LED packages. Furthermore, inductors typically cannot be made to be both efficient and handle a relatively wide range of voltages, and inductive converters generally require significant capacitance to store energy during converter operation. Thus, conventional voltage conversion apparatus based on inductors have a fairly significant footprint when compared with a single or multiple LED packages, and do not readily lend themselves to integration with LED packages.
Capacitive voltage conversion systems present similar challenges. Capacitive systems cannot convert voltage directly, and instead create fixed fractional multiplied or divided voltages. The number of capacitors required is directly related to the product of the integers in the numerator and denominator of the fraction. Since each capacitor also generally requires multiple switches to connect it between the higher voltage power source and a relatively lower voltage load, the number of components increases dramatically as the numerator and denominator increase, with a corresponding decrease in efficiency. If efficiency is a salient requirement, these systems must have practical ratios with a unity numerator or denominator; hence, either the input or output are low voltage at higher current, which effectively decreases efficiency. Thus, efficiency inevitably needs to be compromised at any particular operating voltage to decrease complexity and make simpler fractions.